In recent years, more stringent regulations and appreciation of environmental preservation or safety factors have lead to increased demands for a reduction in the use of petroleum-based materials for printing ink solvents, and a reduction in CO2 emissions. Further, bio solvents are attracting much attention, and among such solvents, the effective utilization of bioethanol, which has minimal environmental impact, holds considerable promise. Conventionally, solvents such as toluene, methyl ethyl ketone and ethyl acetate and the like have been used favorably as printing ink media, but Patent Literature 1 discloses the use of a dispersion of titanium white in which ethanol is the main component of the solvent.
However, printing inks containing an alcohol as the main solvent require that the binder resin is made alcohol-soluble, namely is hydrophilized. As a result, the compatibility between the titanium white, which has a hydrophilic particle surface, and the hydrophilized binder resin can be maintained. However, in the case of an organic pigment such as a copper phthalocyanine cyan pigment for which the surface is hydrophobic, a problem arises in that the compatibility with the hydrophilized binder resin deteriorates, making it difficult to ensure good dispersion stability for the pigment.
Further, when pigment dispersion is problematic using only a binder resin, a dispersant is generally also added to the ink. However, chlorine-based resins such as vinyl chloride-vinyl acetate copolymers, which are conventionally widely used dispersants in solvent-based inks, are insoluble in alcohols. Further, among known non-chlorine-based dispersants, there are no effective dispersants that exhibit satisfactory levels of pigment dispersibility and printability.
On the other hand, alcohol solvents also have other drawbacks, including having slower volatilization rates, and consequently inferior productivity, compared with solvents such as toluene, methyl ethyl ketone and ethyl acetate that are used in conventional solvent-based inks. Accordingly, although providing inferior environmental preservation, in order to maintain good productivity, Patent Literature 2 discloses a method in which an ester-based solvent is used in combination with an alcohol solvent as a printing ink medium.
By using an ester-based solvent in combination with an alcohol solvent, vinyl chloride-vinyl acetate copolymers which are insoluble in the alcohol can be dissolved, and good pigment dispersion stability can be ensured. However, when two solvents having very different volatilization rates are combined, such as ethanol and ethyl acetate, the solvent composition within the ink during the printing process changes on the plate cylinder, with the proportion of the ethanol component that has a slower volatilization rate temporarily increasing, and therefore a problem may arise in which the vinyl chloride-vinyl acetate copolymer precipitates, resulting in inadequate re-solubility, particularly during long runs.
Similarly, even when a non-chlorine-based dispersant is used in combination with an alcohol solvent, ensuring a satisfactory state of pigment dispersion and adequate re-solubility across the entire range of the continually changing alcohol-based solvent composition has proven difficult.